a. Field of the Invention
This invention relates to an improved vehicular power supply apparatus for an automotive vehicle which serves as the power supply of an engine starter when the engine is started.
b. Description of the Related Art
A lead storage battery for an automotive vehicle serves as the power supply of the engine starter and other electrical loads such as the auxiliary equipment of the vehicle. When the engine is started, the lead storage battery supplies the starter with a large current on the order of, say, 100.about.200 A for a short period of time. The engine crankshaft is rotated by the starter torque produced by the resulting power, thereby starting the engine.
While the vehicle is traveling, the lead storage battery is charged by the power produced by a generator mounted on the vehicle which is driven by the engine. The battery is thus charged to make up for the large amount of power consumed in starting the engine.
In a well-known rechargeable lead storage battery, which is a secondary battery, charging and discharging take place owing to the chemical action of an electrolyte and electrodes. Consequently, at discharge, a large amount of current is capable of being discharged in a short period of time, as indicated by the discharge characteristic shown in FIG. 6(a). During charging, however, a current of less than 10 A is supplied over a long period of time to achieve the charging, as indicated by the charge characteristic shown in FIG. 6(b). If a large amount of current were to be supplied, heating and electrode deformation would result.
Accordingly, in vehicles whose users drive only short distances and for short periods of time (as for ordinary commuting), and in vehicles whose users make a large number of starts and stops (as for making deliveries), actual driving time is short while the number of times the engine is started is large. As a result, the electrical power expended by battery discharge during starting cannot be compensated for by sufficient charging, and hence there is the danger that the battery will die, thus making it impossible to start the engine.
A power supply apparatus for automobiles disclosed in the specification of Japanese Utility Model Application Laid-Open (KOKAI) No. 56-146644 includes a series circuit composed of an engine starter and a starter switch, and a series circuit composed of a unidirectional element and a capacitor. The two series circuits are connected in parallel to a DC power source, both ends of the capacitor are connected to a load, and power is supplied to the load.
In this proposed power supply apparatus for automobiles set forth in the above publication, the power supply circuit employs a large-capacity capacitor comprising an electrical double layer capacitor. However, since this large-capacity capacitor is for the load, it is isolated from the starter circuit by a diode during engine start and therefore does not constitute part of the engine starting power supply.
In studies regarding the designing of batteries for automotive vehicles, most emphasis has been placed on the ability to start an engine as a deciding factor of battery capacity. Accordingly, the battery for an automotive vehicle is required to be high in weight and capacity in consideration of the power consumed at during starting of the engine, which demands a large amount of power. On the other hand, since the battery is supplied with power once the engine has been started by the generator for the vehicle which is driven by the engine, a battery of small capacity is thereafter sufficient.